In an image forming device which uses a powder developer constituted by a toner and a carrier, measurement of developer density, namely, a weight mixture ratio of the toner to the carrier (hereinafter, referred to as "toner density"), needs to be performed, further the toner needs to be replenished based on a measured value so as to keep a density of an image properly.
Therefore, conventionally, as a method of measurement for the toner density, an optical measurement method is proposed in which the developer in a developer agitator section is illuminated through a transparent detection window, and the toner density in the developer is measured from quantity of reflected light from the developer.
However, in this optical measurement method, there is a disadvantage that real toner density in the developer can not be measured when the developer adheres onto the detection window.
Further, the measured value varies with variations of amount and bulk density of developer coming into contact with the detection window. In other words, there is a disadvantage that the measured value may indicate improper developer density in consequence of the variations, even if real density is proper.